Scroll devices are well-known in the field of vacuum pumps and compressors. In a scroll device, a movable spiral blade orbits with respect to a fixed spiral blade. The movable spiral blade is connected to an eccentric drive mechanism. The configuration of the scroll blades and their relative motion traps one or more volumes or “pockets” of a gas between the blades and moves the gas through the device. Most applications apply rotary power to pump the gas through the device. Oil-lubricated scroll devices are widely used as refrigerant compressors. Other applications include expanders, which operate in reverse from a compressor, and vacuum pumps. Scroll pumps have not been widely adopted for use as vacuum pumps, mainly because the cost of manufacturing a scroll pump is significantly higher than a comparably sized, oil-lubricated vane pump. Dry scroll pumps have been used in applications where oil contamination is unacceptable. A high displacement rate scroll pump is described in U.S. Pat. No. 5,616,015, issued Apr. 1, 1997 to Liepert.
A scroll pump includes stationary and orbiting scroll elements, and a drive mechanism. The stationary and orbiting scroll elements each include a scroll plate and a spiral scroll blade extending from the scroll plate. The scroll blades are intermeshed together to define interblade pockets. The drive mechanism produces orbiting motion of the orbiting scroll element relative to the stationary scroll element so as to cause the interblade pockets to move toward the pump outlet.
Careful design of the scroll pump is required to provide the close spacing needed for an acceptable compression ratio and yet avoid undesired contact between the scroll blades during operation. Thermal expansion causes component dimensions to vary, both axially and radially. Thus, thermal performance must be considered. Tip seals are typically utilized between the tip of each scroll blade and the adjacent scroll plate. The tip seals may be resilient to accommodate dimensional variations resulting from thermal expansion. The thermal performance of the scroll pump is complicated by the fact that some elements, such as the motor and the crankshaft, can experience significant heating during operation, while other components, such as the external housing, may experience little heating. Further, the scroll pump may be required to operate over a range of temperatures.
U.S. Pat. No. 4,382,754, issued May 10, 1983 to Shaffer et al., discloses a scroll-type apparatus wherein the scroll elements are formed with varying thicknesses along the lengths thereof to accommodate a difference in thermal expansion between the innermost and outermost zones of the apparatus. U.S. Pat. No. 4,490,099, issued Dec. 25, 1984 to Terauchi et al., discloses a scroll-type apparatus wherein the scroll blades are thicker near the center to avoid being affected by dimensional errors of the scroll blades or by thermal expansion. U.S. Pat. No. 4,773,835, issued Sep. 27, 1988 to Machida et al., discloses a scroll-type pump wherein a curve of the scroll blade is offset inwardly or outwardly relative to a set curve so as to prevent formation of a gap between the blades due to thermal expansion of the scroll blades. These patents are directed to the problem of radial expansion but do not address the issue of axial expansion in a scroll-type pump.
Accordingly, there is a need for improved scroll-type pumping apparatus and methods.